Flabellina, chromodoris, halgerda, ovulide; just to name a few of some of the weirdest, intriguing and highly evolved creatures of the ocean, and yet creatures which even those of us who spend a third of our waking day under the sea still know so little about. Last week, during a night dive at Koh Tachai North reef I came face to face with something which I was unable to identify.

I had absolutely no idea what it was. It was about fifteen centimeters in length, dark red in colour and appeared to be a kind of unshelled mollusk moving with impressive rapidity. My customers were as equally mesmerized. This is rare as usually guests seem to want to see either ‘action’ or ‘big things’, and this malleable blob fell into neither category. Back on the boat I consulted the underwater bible to the ‘dark side’, Helmut Debelius’s Field Guide to Nudibranchs and Sea Snails. Page 153 revealed the pleurobranchus forskali, which is a type of notaspidea or sidegill slugs. The passage includes the following rather disturbing description:


These slugs are grazing carnivores, feeding on sponges, tunicates, and other sessile invertebrates.

The passage also mentions that they are ‘voracious predators’ who use their ‘strong jaws’ to ‘feed on a variety of large prey’ and they eat each other. This seems particularly vicious as despite moving faster than most other slugs, ‘fast’ for a slug is still an excruciatingly ‘slow’ pace in which to be eaten. In the past I have witnessed a type of gymnodoris munching away at a fellow slug – head first. This order of courses actually made me wonder whether the ‘eaten’ managed to steal a few retaliatory bites from the ‘eater’ first? Or perhaps it was a self defense mechanism on the part of the ‘eater’ to make sure that the ‘eaten’ didn’t turn around to join in the meal. Either way, this detail along with the description below is an attempt to pull the reader towards the incredible and addictive world of the ‘dark side’; rather than diving to see big fish (which you would need to be blind to miss), search instead for the slugs of the sea.

Now for the Clever Part

In the sea, slugs can generally be catergorised as either snails or ‘nudibranchs’, which literally means ‘naked gills’ in reference to the bushy plume on their bodies through which they breathe. So far about 11,000 species have been identified, (and this does not include the glorious family of flatworms, which will require a whole other article!) Due to their often individual markings, and ability to take on the colorings and even shapes of their food, they are very tricky to identify. However, this hasn’t stopped me from scouring the nudibranch bible and to no avail deciding that I simply must have ‘discovered’ a previously undiscovered slug.

Now what is really remarkable is how nudibranchs have evolved out of their shell, and in the case of the flabellina pteraeolidia ianthina, evolved out of the need to eat! (Commonly found on Richelieu Rock, Koh Tachai Pinnacle, Elephant Head Rock) The removal of the shell has been achievable through the development of a new line of defense – stinging cells which are ingested from prey (which includes stinging hydroids, sea anemones and the spawn of other nudibranchs). The cells are either stored in defense glands along their bodies or secreted. These chemicals are frequently toxic or distasteful, and therefore not a top food choice for a hungry fish. However, their cryptic patterning also helps to make them nearly invisible, which is not just a challenge to the recreational diver to identify, but also for predators to find. If they are spotted then their bright colouring reminds fish of their inedible qualities.

Fortunately, the chances of nudibranchs being eaten before they die of old age are not so high as they have an incredibly short life span, from as little as three weeks. This means that reproduction needs to be efficient and needs to be speedy. Consequently, when they are not busy eating one another, a nudibranch can mate with another member of the species with surprising ease because they are hermaphroditic; they are both male and female. Apparently, they ‘can rarely fertilize themselves’ but just entertaining the notion that there is a small possibility is pretty astounding. When sperm sacs are exchanged the eggs are deposited in a colourful spiral, often on top of their favorite food. This nutritious start to life is used to its full advantage by certain types of facelinidae, which feed on algae rich hydroid coral. They algae is actually ‘farmed’ and then stored for photosynthetic nutrient production, meaning that the algae is able to grow and multiple within the nudibranch’s tissues and it need never eat again!

Ultimately, the moral of this story is: next time your dive guide shows you a blob –get excited. Oh and watch out for those spearing textile cone shells…